The principal objectives of this study are: (1) to evaluate the Wolfe system of breast parenchymal pattern classification for both film-screen and xeromammography, in terms of the relative risks of breast cancer incidence within a period ending 7.5 to 10 years after a negative screening examination; (2) to assess the interrelationships of the Wolfe classification with other breast cancer risk factors; (3) to measure the inter- and intra-observer reliability of the Wolfe classification at different levels of observer expertise. Women enrolled at four Breast Cancer Detection Demonstration Projects will be followed through 1983 by telephone and letter contacts, supplemented by tumor and death registries where feasible, in order to identify cases of breast cancer in initially disease-free women; pathology review of all cases will verify the presence of cancer. The initial negative mammograms for each incident breast cancer case (approximately 600-800 are expected), along with those for a random sample of twice as many controls, will be blindly classified according to the Wolfe scheme, and additional risk factor data will be collected on each subject. The data from this case-control study will provide estimates of relative risks of breast cancer associated with Wolfe class taking into account the effects of the other factors. Reliability of the Wolfe system will be measured for both radiologists and radiology technologists. It is expected that information will be obtained that might contribute to an improved determination of the frequency of mammographic screening.